Initiator with a bridgewire in contact with slurry-loaded pyrotechnic charge at a position of relatively low void formation

ABSTRACT

A pyrotechnic initiator with a bridgewire in contact with a slurry-loaded pyrotechnic charge at a position of relatively low void formation, at an edge of the charge.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to the field of pyrotechnic initiators, and more particularly to an initiator with a bridgewire in contact with a slurry-loaded pyrotechnic charge at a position of relatively low void formation.

[0002] Pyrotechnic initiators have many uses in industrial and consumer applications. One important use is in triggering the inflation of airbags in motor vehicles. Significant efforts have been made in the automotive industry to reduce the cost of manufacturing reliable airbag initiators, yet such initiators must meet demanding performance standards. For example, a firing current having at least a predetermined “all-fire” level and duration (e.g., 800 mA for 2 milliseconds at −35° C.) must resistively generate heat in the bridgewire of the initiator that is reliably (e.g., 99.9999% of the time with at least 95% confidence) sufficient to ignite the initiator's charge. It is also generally required that the application of current up to a predetermined “no fire” level and duration (e.g., 200 mA for 10 seconds at 85° C.) will reliably not result in the bridgewire generating sufficient heat to ignite the charge. The predictability of the all-fire and no-fire levels of an initiator are in significant part determined by the consistency of the charge surrounding the bridgewire, and the consistency of the interface between the charge and bridgewire.

[0003] One advance has been the use of liquids and slurries in loading pyrotechnic charges into the initiators. As shown in U.S. Pat. No. 5,686,691 to Hamilton et al. (which is incorporated herein by reference for its disclosure of slurry-loading except to the extent that it contradicts anything explicitly set forth here), it is known to load a slurry charge into a can and place the can onto a header assembly under a high consolidation force so that the charge consolidates and forcefully presses against the header surface and bridgewire. It is not always desirable; however, to consolidate such charges. For one thing, consolidation tends to require a highly flush glass surface (because the extremely fine bridgewires typically used are easily damaged if pressed against irregularities or voids in the surface of the glass), which generally requires a machining or grinding step.

[0004] It is also known to apply a primer or ignition charge as a slurry droplet without consolidation force, however the formation of voids, which are created by the solvent evaporating from the charge, present a problem in unconsolidated dried slurry charges. Voids are undesirable because compared to the thermal conducting characteristics of the pyrotechnic charge, voids act as thermal insulators. Therefore, voids formed on the bridgewire inhibit ignition, and may lead to an initiator failing to meet the all-fire or no-fire requirement. Voids formed near the bridgewire and covering only a portion of it may result in local hot spots when a small amount of current passes through the bridgewire. These hot spots can ignite the ordnance with much less current than normally required, leading to an initiator that fails to meet the no-fire requirement. Thus, it has been sought to minimize the formation of voids in such unconsolidated charges. Suggested methods of doing so include reducing the depth of the slurry charge (which thus tends to reduce the formation of voids, thus creating a more rigid monolithic mass that has enhanced integrity with the bridgewire that the charge partly or wholly encapsulates), and controlling the composition of the slurry, including its binder and solvent makeup and content, as disclosed in U.S. Pat. No. 5,648,634 to Avory et al., which is incorporated herein by reference. The Avory patent also indicates that such charges may be lightly pressed with a rod.

[0005] It is believed that heretofore the problem of void control has not been addressed by placing the initiator's bridgewire in contact with the slurry at a position of relatively low void formation of the slurry.

SUMMARY OF THE INVENTION

[0006] In accordance with the present invention, an initiator includes a bridgewire that is in contact with a slurry-loaded pyrotechnic charge at a position of relatively low void formation, at an edge of the slurry.

BRIEF DESCRIPTION OF THE FIGURES

[0007]FIG. 1 is a partial side sectional view of an embodiment of an initiator according to the present invention, showing a bridgewire placed at a bottom edge of the slurry in a thin section of dried slurry charge.

[0008]FIG. 2 is a partial side sectional view of an alternate embodiment of an initiator according to the present invention, showing a bridgewire placed at a top edge of the dried slurry charge.

[0009]FIG. 3 is, a partial side sectional view of another alternate embodiment of an initiator according to the present invention, showing a bridgewire placed at both a top and bottom edge of two layers of dried slurry charge.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS¹

[0010] Various initiator configurations can be used, or modified appropriately for use, in the present invention. A suitable initiator for use in the present invention preferably includes a number of features typically found in pyrotechnic initiators, such as are depicted in assignee's co-pending application Ser. No. ______, entitled “Initiator with an Internal Sleeve Retaining a Pyrotechnic Charge and Methods of Making Same,” by Vahan Avetisian et al., (Express Mail No. EU124494073US), which is incorporated herein by reference. For example, a glass-to-metal sealed header assembly is hermetically attached to a charge can, an insulator cup, and a molded insulating body. And, a slurry ignition charge is loaded and dried to form a monolithic solid, preferably with a height that is a small portion of the height of the charge enclosure. An output charge is also preferably loaded (preferably in slurry form) on top of the ignition charge after the ignition charge has dried. As is taught in assignee's co-pending application Ser. No. ______, entitled “Initiator with a Slip Plane Between an Ignition Charge and an Output Charge” by Vahan Avetisian et al., (Express Mail No. EU124494039US), a “slip plane” may also be provided between the ignition charge and output charge so as to prevent the transmission of forces from the output charge into the ignition charge.

[0011] Turning to FIG. 1, a suitable header assembly may include an eyelet 20 that is typically made of a metal such as 304L stainless steel, and is generally cylindrical with a passage defined through it to permit a feedthrough to be created by the hermetic sealing of the glass 22 and the center pin 30 therein. The glass 22 may preferably consist of sodium aluminosilicate, barium alkali silicate, or other well-known glasses. A bridgewire 28 extends from a radially extending surface of the center pin 30 to a radially extending surface of the eyelet 20. The bridgewire 28 may be formed from a high resistance metal alloy such as platinum-tungsten or “NICHROME” nickel-chromium alloy. The bridgewire 28 has flattened opposite end portions that are fixed to the center pin 30 and the eyelet 20 by electrical resistance welds. These opposite end portions of the bridgewire 28 become flattened (not shown) under the pressure applied by the welding electrodes that are used to form the resistance welds. A suitable header, slurry charge composition, and bridgewire for use in the present invention may be such as are described in assignee's co-pending application Ser. No. ______, entitled “Axial Spin Method of Distributing Pyrotechnic Charge in an Initiator,” by Marius Rosu, (Express Mail No. EU124494056US); Ser. No. ______, entitled “Initiator with a Bridgewire Configured in an Enhanced Heat-Sinking Relationship,” by Vahan Avetisian, (Express Mail No. EU124494060US); and Ser. No. ______, entitled “Initiator with an Internal Sleeve Retaining a Pyrotechnic Charge and Methods of Making Same,” by Vahan Avetisian et al., (Express Mail No. EU124494073US).

[0012] In the embodiment of the present invention shown in FIG. 1, a slurry ignition charge 24 is placed in a layer on the surface of the header around the bridgewire 28 in a heat-receiving relationship with the bridgewire 28, and is allowed to dry to form a monolithic solid. The bridgewire 28 is located at a bottom edge of the slurry ignition charge 24, where the slurry experiences the least formation of voids 32, because the slurry tends to dry at its edges first, leaving voids interior to the dried edges. To also minimize the depth of slurry at the location of the bridgewire while still retaining a specified amount of igniter charge, the axial spinning technique taught in assignee's co-pending application Ser. No. ______, entitled “Axial Spin Method of Distributing Pyrotechnic Charge in an Initiator,” by Marius Rosu, (Express Mail No. EU124494056US), may be applied such that a depression is created in the ignition charge in the region of the bridgewire. It is generally preferable that the depth of the igniter charge above the bridgewire be no more than fifty times the thickness of the bridgewire. It is also noted that the placement of the bridgewire flush against the header surface likely helps to reduce the tendency of any electrostatic discharges occurring between the eyelet and the portion of the bridgewire that is slightly raised above the eyelet, and between the pin and the portion of the bridgewire that is slightly raised above the pin.

[0013] In the embodiment of the present invention depicted in FIG. 2, reduction in exposure to voids 32 (and cracking) is also attained. In this embodiment, instead of being at the bottom of the ignition charge layer and between it and the header surface, the bridgewire is at the top of the ignition charge, which is distributed annularly in a recess above the glass 22 of the header. Because the evaporation occurs most rapidly at the top, where the slurry charge is exposed to air during drying, the region of the slurry adjacent the bridgewire is least likely to contain voids 32. Some headers having recessed glass insulators that are generally applicable to this embodiment are described in U.S. Pat. No. 5,821,446 to Chatley Jr., which is incorporated herein by reference, and in assignee's co-pending application Ser. No. 09/733,548, entitled “Recessed Glass Header for Pyrotechnic Initiators” and filed on Dec. 7, 2000.

[0014] In a third embodiment depicted in FIG. 3, the features of both of FIGS. 1 and 2 can be combined. Thus, in this embodiment bridgewire 28 is sandwiched between the edges of two layers of dried slurry charge. To ensure that both of those edges contain a minimum of voids 32, such an embodiment should be made by loading and drying layer 1, and then loading and drying layer 2.

[0015] A preferred embodiment of a pyrotechnic initiator including a bridgewire in contact with a slurry-loaded pyrotechnic charge at a position of relatively low void formation has thus been disclosed. It will be apparent, however, that various changes may be made in the form, construction, and arrangement of the parts without departing from the spirit and scope of the invention, the form hereinbefore described being merely a preferred or exemplary embodiment thereof. Therefore, the invention is not to be restricted or limited except in accordance with the following claims. 

What is claimed is:
 1. An initiator comprising: a) a header assembly including a top, an eyelet, a glass insulator, and a first pin; b) an exposed electrical initiating element at said top of said header assembly; c) a first ignition charge adjacent to said top surface of said header assembly, said ignition charge being adjacent to and in heat-transferring relationship with said electrical initiating element; and, d) said first ignition charge being a dried slurry, said first ignition charge being in contact with said electrical initiating element at an edge of said first ignition charge.
 2. The initiator of claim 1, wherein said electrical initiating element is a bridgewire.
 3. The initiator of claim 2, wherein said first ignition charge is above said bridgewire.
 4. The initiator of claim 3, wherein said glass insulator has a convex annular top surface.
 5. The initiator of claim 1, wherein said glass insulator has a flat top surface that is substantially flush with said top surface of said header assembly.
 6. The initiator of claim 3, wherein said bridgewire is in intimate contact with said glass insulator.
 7. The initiator of claim 6, wherein said bridgewire is a flattened bridgewire.
 8. The initiator of claim 2, wherein said first ignition charge is further in contact with said electrical initiating element at a position of said first ignition charge that has a height no greater than fifty times the diameter of said bridgewire.
 9. The initiator of claim 2, wherein said first ignition charge is below said bridgewire.
 10. The initiator of claim 9, further comprising a second ignition charge that is adjacent to and in heat-transferring relationship with said electrical initiating element, said second ignition charge being a dried slurry, said second ignition charge being in contact with said electrical initiating element at a bottom edge of said second ignition charge.
 11. The initiator of claim 10, wherein said first ignition charge is further in contact with said electrical initiating element at a position of said first ignition charge that has a height no greater than fifty times the diameter of said bridgewire, and said second ignition charge is further in contact with said electrical initiating element at a position of said second ignition charge that has a height no greater than fifty times the diameter of said bridgewire.
 12. The initiator of claim 3, wherein said first ignition charge is not pressed against said top of said header assembly by a pressure of more than 50 psi.
 13. The initiator of claim 12, further comprising an output charge on top of said first ignition charge.
 14. The initiator of claim 13, further comprising an intermediary slip plane between said first ignition charge and said output charge.
 15. A method of making an initiator, comprising the following steps: a) providing a header assembly including a top, an eyelet, a glass insulator, and a first pin; b) providing an exposed electrical initiating element at said top of said header assembly; c) loading a first ignition charge in the form of a slurry, adjacent to said top surface of said header assembly and in heat-transferring relationship with said electrical initiating element; and, d) drying said first ignition charge so that said first ignition charge remains in heat-transferring relationship with said electrical initiating element at an edge of said first ignition charge.
 16. The method of claim 15, further comprising the step of rendering the portion of said first ignition charge adjacent to said electrical initiating element the thinnest portion of said first ignition charge.
 17. The method of claim 16, wherein said step of rendering comprises axially spinning said header assembly while it is being loaded with said first ignition charge.
 18. The method of claim 15, further comprising the step of centrifuging said header assembly while it is being loaded with said first ignition charge.
 19. The method of claim 15, wherein step c) comprises loading said first ignition charge into a recess in said top of said header, the method further comprising the steps of loading a second ignition charge onto said top of said header assembly, and drying said second ignition charge.
 20. The method of claim 19, further comprising the step of selecting slurry compositions for said first and second ignition charges that result in minimal void creation in said edges of said first and second ignition charges upon said steps of drying. 